In reciprocating engines and other devices, a piston may reciprocate within a cylinder to produce useful power. In a typical internal combustion engine, for example, one or more pistons may be housed within one or more corresponding cylinders, with each piston connected to a crankshaft by a connecting rod. At the end opposite the crankshaft, each cylinder may be closed (e.g., by the engine cylinder head), with the piston thereby defining (along with the cylinder) a combustion chamber. Various valves and other mechanisms may then control the in- and out-flow of air and fuel. When a piston is fully displaced into the cylinder (and away from the crankshaft) the piston may be considered to have reached top dead center (“TDC”). As such, TDC may generally be viewed as the point within a piston's cyclical motion at which the nominal maximum compression of gas within the cylinder (and the minimum combustion-chamber volume) has been obtained. Likewise, when a piston is fully retracted away from TDC (and toward the crankshaft) the piston may be considered to have reached bottom dead center (“BDC”). As such, BDC may generally be viewed as the point within a piston's cyclical motion at which the nominal minimum compression of gas within the cylinder (and the maximum combustion-chamber volume) has been obtained, It will be understood, however, that other configurations may be possible. Therefore, it may also be useful to consider TDC and BDC as opposite orientations for a piston at which, for both TDC and BDC, a normal force applied to the piston face is directed, via the associated connecting rod, straight along the main axis of the relevant cylinder or straight into an associated crankshaft.
During engine operation (or as the engine is otherwise motored), the pistons may travel along the path of various piston strokes, each of which may be considered as including the path of travel of a piston between TDC and BDC (or vice versa). In this light, it may be useful to consider a reciprocating piston as having two main categories of strokes—an “up-stroke,” during which the piston is progressed in a direction from BDC toward TDC; and a “down-stroke,” during which the piston is progressed in a direction from TDC toward BDC. In certain engines, further distinction may be made, with respect to powered and other strokes. For example, in a four-stroke engine, a first up-stroke may compress air within the combustion chamber, a first down-stroke may be driven by combustion of fuel within the cylinder (and the associated expansion of the contained air and combustion products), a second up-stroke may force air and combustion products out of an exhaust valve, and a second down-stroke may draw new air into the cylinder through an intake valve, in order to re-set the engine for the next cycle.
For various reasons, it may be highly useful to determine the exact (or near-exact) location of TDC for the various pistons of a reciprocating engine (or other cylinder-piston systems). For example, ignition timing for an internal combustion engine may often be specified with respect to TDC (e.g., so many degrees before or after TDC). Because precise ignition timing can play an important role in controlling engine dynamics and the corresponding composition of exhaust emissions, it may be useful to know the location of TDC with a high degree of precision. It may be difficult, however, to determine the location of TDC with high precision based upon manufacturing specifications alone. For example, even with highly precise manufacturing, the permitted manufacturing tolerances of various parts can combine to introduce relatively large uncertainty with regard to the actual TDC position of any given piston. As such, although an expected TDC position may be identified (e.g., based on manufacturing specifications, visual inspection, and so on), this expected TDC position may sometimes vary from actual TDC by 1.5 degrees or more.
In current practice, linear displacement instruments are often utilized to measure TDC before the engine cylinder head is installed on the engine block. For example, an engine block may be securely mounted, and a linear displacement instrument (e.g., a linear encoder) may be securely fixed with respect to the engine block, with a probe extended into contact with the head of a piston that is not at TDC. The piston may then be advanced along its cyclical path (e.g., towards and then past the expected position of TDC), which will displace the probe accordingly. In such a set-up, the minimum extension of the probe during the progression of the piston may be viewed as corresponding to TDC. If the position of the piston is recorded (e.g., via a mechanical, magnetic, or other sensor associated with the crankshaft or a related gear) and correlated with the linear displacement measurements, the approximate position of TDC with respect to crankshaft rotation (or similar reference) may be then be recorded. This method may present various disadvantages, however, including somewhat limited accuracy, the need to securely fix the engine in place to execute the testing, and the general inability to conduct the testing with the cylinder head in place.